Sloping ceiling insulation

[cycloneuk]

We live in a mid-terrace house, and the upstairs ceilings at the front and back slope for the last 16 inches or so. The problem is that the ceilings are old lath and plaster, so they’re pretty thin, and these sections have no insulation. The rest of the loft is boarded, although not very well. Under the boards there’s 100mm of insulation. I was going to just roll another 170mm on top, as I like the idea of being able to access the loft and could just roll sections back if needed.

But I think the best solution is probably to take all the boards up, put another 170mm on top, and then use loft legs for a storage space. The gap between the sloping sections and the slates is about 100mm. I can’t see how I could get anything down between the joists in those sections, and it probably wouldn’t fit very well anyway. We do get mould spots on these parts of the ceiling, although less since we installed a Drimaster PIV.

What about screwing some insulated plasterboard onto the sloping sections and then skimming just that area? I don’t want to lose too much ceiling height, but any extra thickness is better than nothing, right? It’s the only idea I can think of. These old terrace houses are an absolute nightmare when it comes to insulation—we’ve got solid walls and separate damp issues downstairs as well.

 

[cycloneuk]

Because the ceiling is lath and plaster, the loft side has ridges where the plaster comes through the laths. I wouldn’t be able to get the boards to sit flush, so there would be an air gap between the board and the ceiling, which I’m guessing would drastically reduce effectiveness but maybe better than nothing.